188 SUPERHEATED STEAM IN MARINE PRACTICE. 



ADVANTAGES TO BE GAINED BY USING SUPERHEATED STEAM. 



63. What will be the advantage, and what has the shipowner to find of in- 

 terest and value, by installing superheaters? Briefly they appear to be: — 



1st. Fuel economy. 



2d. Water economy. 



3d. Reduced valve leakage. 



4th. Increased boiler capacity. 



5th. Increased revenue cargo space. 



64. Fuel economy cannot be any less important in the future than it is to-day. 

 If any change in fuel costs takes place, they will go up rather than down, and, as 

 they increase, the question of fuel saving will become more pressing. With the 

 use of reciprocating engines, superheating presents practically the only source of 

 further pronounced economies that is left to the engine designer. 



65. Plate 95 shows, in tabular form, all of the tests which the writer has 

 been able to find. We hear railroad men talking eloquently and continually of 

 fuel economies of 25 per cent, 30 per cent, and in some cases 35 per cent. To ex- 

 pect these in marine practice is, of course, out of the question. The marine engine 

 as it stands to-day is a much more efficient and economical plant than the railway 

 man has had available. Superheating, therefore, has permitted a higher percentage 

 of economy on the railways than is possible in marine practice. It may, however, 

 be concluded that, assuming 150° of superheat at the high-pressure chest, the follow- 

 ing fuel economies may be confidently expected in average practice: — 



Compound engines, 18 per cent. 

 Triple expansion engines, 15 per cent. 

 Quadruple expansion engines, 8 per cent. 



66. Many of the tests shown on Plate 95 are in excess of the figures just 

 quoted. In well-designed and well-built plants operating efficiently with saturated 

 steam, these large economies may not always be obtained. 



67. Water economy is obtained wherever we get fuel economy. In percentage, 

 the economy in steam will be greater than in fuel. An 8 per cent fuel economy by 

 superheating gives 10 per cent and upward of steam economy. It will be readily 

 appreciated that an economy of steam of this amount is directly of advantage in 

 reducing : — 



1st. Boiler maintenance, as the quantity of water to be evaporated is decreased. 



2d. Work demanded of the evaporators. 



3d. Work required of feed water pumps and condensers. 



68. These, directly and indirectly, are advantages that should be taken into 

 account. 



69. Leakage past piston and valve rings is a great source of loss. It has been 

 demonstrated that water leaks more readily than steam, and that the heat loss is 

 greater. This accounts, in part, for a greater water than fuel economy in using 

 superheated steam. Professor Perry says: — 



